Of the 50(?) or so stock maps that exist, I can name only two, Badwater and Frontier, that were played "just about every night" as of a week or two ago. We play on Harvest, Upward, Thunder Mountain, and several other maps pretty frequently, but unless the crazy resurgence in Degroot Keep you're describing has only happened recently, we hardly play it "every single night". Hell, I'd estimate that the server rolls through about 5-6 maps on a typical night depending on what's played. The only monotony I directly recall with regard to map votes is Frontier magically winning more than I'd care to play it.
If map voting doesn't automatically happen at the end of every single map, nominating will only even matter if half the server chooses to RTV off the current map. I don't think we'd see nearly as much abuse of the system if that was the case.
As for the "recently played" list of maps that can't be nominated, I'd completely agree with lengthening it enough to prevent the same group of maps from being played over and over, but sometimes everyone just wants to play cplrtf_dustrushbadbowlgoldwater.